Default key is "Enter" (OR "Return" if you're on a Macintosh OS with Bootcamp).

If that does not work, go into your settings look through the key assignments to check which key it is assigned to.

I'd check you're hitting the right Enter button - its only the one on the far right past the keypad. If playing on a laptop without a sufficiently big keyboard you may be out of luck!

I'm on a laptop with numeric pad, and both enter keys work fine

player is on 1st person server?